vfs: add helpers to get root and pwd

Add three helpers that retrieve a refcounted copy of the root and cwd
from the supplied fs_struct.

 get_fs_root()
 get_fs_pwd()
 get_fs_root_and_pwd()

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index b87a63b..1b31c13 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -1835,13 +1835,8 @@
 	context->names[context->name_count].ino  = (unsigned long)-1;
 	context->names[context->name_count].osid = 0;
 	++context->name_count;
-	if (!context->pwd.dentry) {
-		read_lock(&current->fs->lock);
-		context->pwd = current->fs->pwd;
-		path_get(&current->fs->pwd);
-		read_unlock(&current->fs->lock);
-	}
-
+	if (!context->pwd.dentry)
+		get_fs_pwd(current->fs, &context->pwd);
 }
 
 /* audit_putname - intercept a putname request